There is a growing need for technical development to recover valuable metals, especially noble metals (Pt, Pd, Rh, Ru and Ir) in the smelting of nonferrous metals such as copper.
To determine the appropriate method for recovering noble metals, it is important to grasp the behaviors of the noble metals in the smelting process. For example, it is important to examine the mass balance of noble metals from a flash furnace to an electrolytic bath. However, such grasping of the behavior has been difficult so far since their amounts are often very low, i.e., equal to or less than the order of μg/g.
Therefore, there is a need for a method for analyzing noble metals with high sensitivity. To attain the object, an analysis method having the determination limit in the order of 0.01 g/t (0.01 μg/g) is required.
Conventionally, a dry assaying method or nickel mat method has been used for the analysis of noble metals. These methods are excellent at decomposition of a large amount of sample, concentration and separation from other components, and sensitivity, and have been used for the analysis of minute amounts of gold, platinum and palladium contained in mineral raw materials. However, these methods don't satisfy the aforementioned determination limit level, therefore there has been a need for a different analysis method.
A high-frequency plasma mass spectroscope using inductively coupled plasma (ICP) or microwave induced plasma (MIP) has been known as an apparatus for analyzing minute amounts of elements with high sensitivity. A high-frequency plasma mass spectroscope analyzes isotopes or elements by ionizing target elements contained in a dissolved sample with high-frequency plasma, feeding the generated ions to a mass spectrometer, and counting the number of ions in mass/charge number (m/z) of the target elements.
A high-frequency plasma mass spectroscope primarily comprises an ionizing section equipped with a plasma torch for ionizing target elements with high-frequency plasma, an interface section equipped with a sampling cone and a skimmer cone which are differentially pumped to feed the ions generated in the HFP at atmospheric pressure to a mass analysis section under a high vacuum condition, and the mass analysis section for mass analysis of the generated ions.
Conventionally, it is known that the optimal distance between the sampling cone and the skimmer cone, at which the highest sensitivity to the target ions is observed, varies over time due to the deterioration of those components. In addition, it is also known that depending on the distance between the sampling cone and the skimmer cone, the target ions are oxidized, thereby complicating the mass spectrum and causing analysis errors. Japanese unexamined patent publication No. 9-129174 discloses a high-frequency plasma mass spectroscope comprising a means for changing the distance between a sampling cone and a skimmer cone. This patent publication states that it can automatically determine the position where the highest sensitivity can be obtained for the ions of a target element. In addition, it can also automatically determine the position where the generation of oxides is suppressed by successively changing the distance between the sampling cone and the skimmer cone while monitoring the ionic strengths for both the target element and its oxide.